THE INFLUENCE OF CUT-BANK CHARACTERISTICS ON STABLE CHANNEL WIDTH IN PAIRED FOREST/MEADOW, 3RD ORDER, MIXED-BED, PENNSYLVANIA PIEDMONT STREAMS

In a study of four mixed-bed streams in the Pennsylvania Piedmont, I observed a dramatic difference in channel width between reaches of the streams that flow through a forested riparian zone and reaches of the same streams that flow through a non-forested riparian zone. Some might assume that meadow streams are narrower because their banks are easier to erode. Our results show that cut-banks in meadow reaches actually erode 6.7 times faster than those in the forest reaches (Allmendinger et al., 2001). The meadow reaches erode faster, even though they are measurably stronger than their forested counterparts. The average vane shear value for the meadow reaches is 43.1 kPa (s.d.: 23.0 kPa) while the average value for the forested reaches is 23.0 kPa (s.d.: 18.9 kPa). However, cut-banks in meadow reaches erode at 0.7 m/yr (s.d.: 0.6 m/yr), while those in forested reaches erode at 0.1 m/yr (s.d.: 0.2 m/yr). Cut-banks in the meadow reaches contain more coarse sediment than those in the forested reaches, which ought to make them easier to erode. Sediment samples show that cut banks in the meadow reaches are 10% gravel, 44% sand, and 46% mud, while those in the forested reaches are 13% gravel, 35% sand and 52% mud. Cut-banks in the forested reaches are better armored than those of the meadow reaches. On average, 3.6% of the surface area of the cut-bank in the meadow reach has large roots beneath their surface (s.d.: 2.6%) while 32.4% of the surface area of the cut-bank in the forested reach has large roots beneath their surface (s.d.: 4.3%). Small roots are more abundant in the meadow reaches than in the forested reach. However, they appear only to influence the upper 20 cm. of the cut bank. Despite the surfeit of coarse-grained sediment and lack of root armor, cut-banks in the meadow reaches are stronger than those in the forested reaches. Even though the meadow banks are stronger, they still erode at higher rates. This implies that erosion rates may not depend on characteristics of the cut bank itself. Rather, it appears that cut-bank erosion may be caused by flow constriction as a result of deposition on the convex bank.